The recovery of hydrocarbons, especially oil, frequently involves the injection of fluids into the reservoir to either force or drive the hydrocarbons from one location to another, as in flooding operations, or even more basically stated, to improve the flow of the hydrocarbons to the production well as in various stimulation operations. Carbon dioxide and other fluids, including water and steam, are frequently injected for such purposes, particularly for the recovery of oil.
A discussion of the problems encountered with the use of injected carbon dioxide is presented in an article entitled Reservoir Application of Mobility Control Foams in CO.sub.2 Floods, of the Society of Petroleum Engineers/U.S. Department of Energy paper SPE/DOE 12644, pp 159 to 167. Foams and surfactants are frequently used for retarding the formation of viscous fingers durng carbon dioxide floods.
A known method of reducing the flow of water is described in U.S. Pat. No. 3,762,476 wherein a first aqueous polymer solution selected from the group consisting of polyacrylamide, a partially hydrolyzed polyacrylamide, a polysaccharide, a carboxymethylcellulose, a polyvinyl alcohol, and polystyrene sulfonate, is injected into a subterranean formation. Thereafter, a complexing ionic solution of multivalent cations and retarding anions, and which also comprises aluminum citrate, is injected into the subterranean formation. The multivalent cations are selected from the group consisting of Fe(II), Fe(III), Al(III), Ti(IV), Zn(II), Sn(IV), Ca(II), Mg(II), Cr(III), and the retarding anions are selected from the group consisting of acetate, nitrilotriacetate, tartrate, citrate, phosphate. Brine is then injected followed by a second slug of an aqueous polymer solution which can be the same or different from the first aqueous polymer solution. In any event, the complexing ionic solution of multivalent cations and retarding anions is capable of gelling both the first and second aqueous polymer solution.
Water produced from a wellbore can come from the infiltration of naturally occuring subterranean water as described above, or the water can come from injected water put into the formation in those hydrocarbon recovery processes which utilize waterflooding. U.S. Pat. No. 4,098,337 discloses a method for forming a hydroxymethylated polyacrylamide gel, in situ, to reduce the permeability of a thusly treated zone where the waterflood method of oil recovery is employed. In this case the gel was formed in situ by the injection of an aqueous polyacrylamide solution and an aqueous formaldehyde solution.
Although polyacrylamide-based gels can be effective in retarding water production or flow in some subterranean formations, polyacrylamide-based gels will not be stable or effective in all formations. In general, polyacrylamide-based gels will work satisfactorily in formations having a temperature below about 65.degree. C. Above about 65.degree. C., polyacrylamide-based gels become very sensitive to hardness of the brines, especially where hardness is above about 1000 ppm. The hardness of the water becomes a more detrimental factor the higher the temperature, thus for very hot regions even low hardness levels can render many gels ineffective. Formations which have a higher temperature, hardness, or total dissolved solids content above the aforementioned ranges usually are not capable of being succesfully treated with polyacrylamide-based polymers except for a relatively short period of time.
In many hydrocarbon producing wells temperatures of 80.degree. C. or higher are often encountered. Formation waters frequently have hardnesses which exceed 1000 ppm. It is therefore desirable to develop a gel which can be used to retard or block the flow of water in subterranean formations having a temperature of 65.degree. C. or higher, and a water hardness of 1000 ppm or higher.
In other flooding operaions, rather than water, other fluids can be used. Some fluids which are frequently used are carbon dioxide and steam. Carbon dioxide is also used in other treating methods such as "Push and Pull" operations, sometimes referred to as "cyclic carbon dioxide injection" or "Huff and Puff" operations, where a production well is injected with carbon dioxide for several days and then produced for a month or so result in channels being formed which if not blocked will result is an inefficient carbon dioxide treating operation due to loss of the gas into channels which drain into nonproductive parts of the reservoir. Because many of the existing gels degrade rapidly at elevated temperatures, polymers such as polyacrylamides are generally not satisfactory. Other fluids such as steam can also be used in push and pull operations.
Flooding operations using carbon dioxide as the drive fluid frequently experience a loss of drive fluid to nonproductive parts of the reservoir because of greater ability of the gas to dissipate into such channels as opposed to liquids. Loss of drive gases in carbon dioxide flooding operations and carbon dioxide in CO.sub.2 stimulation methods is more difficult to prevent because the flow channels responsible for such losses can be very small in diameter or width thereby making it very difficult to fill such channels with a blocking agent. Some viscous plugging substances, even though they may have the desired stability at higher temperatures, are not able to penetrate and effectively fill narrow channels, particularly as such channels become more distant from the wellbore.
Thus there is a need for plugging agents which can be formulated to penetrate deeply into the formation. The use of this invention addresses this problem and provides polyvinyl alcohol based gels which can be tailor made to the particular problem at hand and which can overcome many of the shortcomings of prior art plugging agents and gels.
Polyvinyl alcohol gels have been used to protect well casings from corrosion. U.S. Pat. No. 2,832,414 discloses such a method wherein an aqueous solution of a water soluble polyvinyl alcohol which is capable of forming a gel if maintained in a quiescent state, is pumped into the annular space between the casing and the wall of the bore hole. After allowing the polymer to remain quiescent over a period of time a gel is formed. The thusly formed gel preventsthe intrusion of formation water into the annular space thereby reducing corrosion of the metal casing. Apparently, no crosslinking agent is employed and for that reason is not believed that this particular gel would be useful for plugging channels or fractures on a permanent bases. Furthermore, in U.S. Pat. No. 2,832,414 the gel is used to fill a relatively large but stagnant cavity compared to the volume of a typical channel in a subterranean formation associated with loss of carbon dioxide.
Studies of the macroscopic changes in polyvinyl acetate gels that occur upon removal from swelling equilibrium with isopropyl alcohol were reported in the Journal of Colloid and Interface Science, Vol. 90, No. 1, November 1982, pages 34 to 43. These studies were conducted using films of gels having various degrees of crosslinking and polymer concentration. The polyvinyl acetate gels were formed from precursor polyvinyl alcohol gels that were crosslinked with glutaric dialdehyde which were then converted to acetate gels by polymer homologous acetylation.
U.S. Pat. No. 3,265,657 discloses a process for preparing an aqueous polyvinyl alcohol composition, which remains fluid for at least a few seconds after preparation and spontaneously gels thereafter. The gel is formed by contacting a gelable fluid aqueous polyvinyl alcohol solution with a hexavalent chromium compound and a reductive agent to convert Cr (VI) to Cr (III). The compositions are used to produce foams suitable as insulating, acoustical, and packaging materials. The gels are crosslinked with chromium, not an aldehyde.
U.S. Pat. No. 3,658,745 discloses a hydrogel which is capable of significant expansion upon cooling in water and reversible shrinking upon heating which comprises a crosslinked acetalated hydrogel formed by reacting a polyvinyl alcohol previously dissolved in water and a monaldehyde and dialdehyde. The hydrogels are alleged to have sufficient crosslinking to prevent inhibition of macromolecular materials such as proteins but not the imbibition of micromolecular materials such as low molecular weight water solutes. These hydrogels are alleged to be useful for dialytic purification when pure water is added to the macromolecular solution after each cycle. Apparently these particular hydrogels are able to absorb and desorb water and microsolutes with alternate cooling and heating cycles. Apparently a major amount of shrinkage of these gels occurs upon slight heating such as from 12.degree. to 37.degree. C. which indicates that these gels would have little value for blocking carbon dioxide and other fluids, including water, in subterranean formations, especially at temperatures of 37.degree. C. or higher.